Super-absorbent polymers have already been known for decades. Such super-absorbent polymers can absorb up to many times their own weight in liquid and are used for diverse different applications. Super-absorbent polymers are generally based on acrylamide or acrylic acid as monomer component, which is cross-linked with a small quantity of a cross-linker. At the location where this cross-linker is incorporated there is created a cross-link between the polymer chains, so that a network results. The polymer chains contain a large number of carboxyl (COOH) groups. Through contact with water or an aqueous liquid, hydrogen atoms will be released and negatively charged carboxylate (COO−) groups will be formed. These carboxylate groups are mutually repellent, wherein the cross-linked polymer as it were forms a three-dimensional network in which the water molecules can be captured.
Super-absorbent polymers can be manufactured in diverse ways. U.S. Pat. No. 6,403,674 for instance describes a method for manufacturing super-absorbent polymers wherein there are provided a first monofunctional monomer, such as acrylic acid, and a second multifunctional monomer, which can be dissolved in each other so that no further solvent is required. The monomer components are mixed together, whereafter they are polymerized under the influence of a suitable energy source.
It is known that the absorption capacity of super-absorbent polymers increases further when the polyacrylic acid is neutralized with a base such as NaOH. The sodium ions here neutralize the negatively charged carboxylate groups. A large amount of moisture can be absorbed via osmosis. The sodium ions are herein hydrated, whereby the distance to the carboxylate groups increases. The free carboxylate groups will once again repel each other, whereby the chains will begin to stretch more such that even more space is created for the absorption of extra moisture. Sodium polyacrylate is generally manufactured by neutralizing polyacrylic acid in an aqueous environment, whereafter the water must be removed, as described in for instance WO 98/52979.
Due to the great number and the diversity of possible applications of super-absorbent polymers, there is a continual demand for improved methods of manufacturing super-absorbent polymers with a high absorption capacity.